Electrical cables are typically used to pass signals between antennas and test equipment in an antenna test range or test room. An electrical cable that passes a signal between two separate locations is subject to mechanical and/or environmental influences such as temperature, motion, pressure, humidity, and deformation, etc. that affect the electrical characteristics of the cable. The accuracy of measurements of the electrical parameters of signals carried by a cable is undesirably affected by such environmental influences on the cable. Inaccurate measurements of cable signals result in inaccurate conclusions about devices connected to the cables. Such environmental influences degrade the accuracy of measurement data acquired via the measurement cable unless the measurement error induced by the cable variation is minimized prior to the measurement data acquisition or the measurement error is eliminated by post-processing of the measurement data.
For example, a planar near-field measurement system conducts precise measurements of the characteristics of an antenna-under-test ("AUT") by moving a probe antenna along an x-y plane located approximately parallel to the wavefront of the AUT to conduct a series of discrete measurements of the AUT. Similarly, a spherical near-field measurement system conducts antenna measurements by moving the AUT in both azimuth and elevation to scan the fixed probe antenna. For either implementation, a near-field measurement system utilizes a moving antenna to conduct antenna measurement operations. In general, the moving antenna is connected to stationary measurement instrumentation by at least one measurement cable, for example, a test signal cable. However, the motion of the moving antenna disrupts the phase and amplitude stability of the test signal cable by flexing the test signal cable. Similarly, changes in the ambient temperature of the test signal cable corrupt the accuracy of a test signal carried by the cable.
Measurement errors induced by environmental influences on the test signal cable cause post-processing errors during the transformation of the near-field measurement data to far-field data and, consequently, introduce errors for the measurement of the characteristics of the AUT. In particular, these errors degrade the accuracy of the measurement of low sidelobe levels for the AUT because sidelobe measurements require extremely accurate phase measurements.
A variety of methods have been proposed to reduce the measurement errors attributable to cable variations. Typically, cable variation errors are minimized for measurement systems, specifically near-field measurement systems, by carefully selecting the type of cable carrying a measurement signal such as the test signal. For example, a flexible coaxial cable, such as a polyetrafluoroethylene (PTFE) cable, is preferred for the test signal cable. This nearly phase stable cable is typically routed along the structure that supports the moving antenna. Careful routing controls the bending radius of the cable and ensures a stable cable location as a function of moving antenna position. The placement of the flexible cable along the support structure requires careful attention to avoid placing the cable in a state of tension or compression at any point along the cable path of motion for the moving antenna.
Nevertheless, the use of a PTFE cable as the test signal cable for a near-field measurement system is complicated by the requirement for a complex mechanical structure to control cable flexure. Furthermore, the effective cable lifespan for a nearly phase stable cable is limited by the flexing of the cable occurring during the movement of the moving antenna.
Other approaches for minimizing the cable variations associated with a test signal cable include the use of semirigid coaxial cable that is typically supported by a set of articulated support arms which are connected to the moving probe antenna in a planar near-field measurement system. A rotary joint having a known phase stability characteristic is typically used to bridge the joint at each articulated arm. Alternatively, flexible coaxial cable jumpers or a loop of thin diameter semirigid cable have been utilized to bridge the joints and provide a continuous signal path between the moving antenna and the measurement instrumentation. Likewise, a combination of semirigid coaxial cable and rotary joints also is utilized to bridge the moving components associated with a turntable that supports and rotates the AUT in a spherical near-field measurement system.
A semirigid cable remains fixed relative to the motion of the moving antenna, thereby minimizing any motion-induced errors in the test signal path. However, typical semirigid cables have a relatively high temperature coefficient of approximately 80 parts per million per degree (ppm/degree) Centigrade, rendering them susceptible to temperature induced errors. For precise measurements, semirigid cables have been found to exhibit less than optimum phase stability during changes in the ambient temperature upon the cable.
It is expected that fiber optic cables will be used in the future to transmit test signals between the instrumentation and the moving antenna. However, fiber optic cable based systems are still in the development stage. At present, the phase stability of fiber optic cables varies widely, and is similar to that of PFTE cables. Consequently, the performance of fiber optic cables does not offer a sufficient advantage over PFTE cable to justify the additional expense of the fiber optic transmitters and receivers required for a fiber optic implementation of a test signal cable.
Measurement errors induced by cable variation during antenna system and network system measurements, including near-field measurements, also can be directly measured and thereby provide data useful in compensating for such errors. One technique for the measurement of phase error induced by cable variation is described in "Planar Near-Field Measurements" by A. Newell of the National Institute of Standards (NIST), pages 34-35, dated June 1985. The Newell technique introduces a calibration signal at a first end of a test signal cable and inserts a significant mismatch by use of a stub tuner at the other end of the test signal cable. The mismatch generates a signal reflection of the calibration signal; the reflection returns to the first end of the cable and is measured by a microwave bridge. The microwave bridge compares the calibration signal to the reflection signal to determine phase errors influenced by environmental effects.
An alternative method for correcting phase errors caused by the flexing of a test signal cable is described in "A New Method for Correcting Phase Errors Caused by Flexing of Cables in Antenna Measurements", by J. Tuovinen, A. Lehto, and A. Raisanen, IEEE Trans. Antennas and Propagation, Vol. 39, No. 6, June 1991, pages 859-861. Similar to the Newell technique, the measurement system measures the phase of a signal passing twice the length of a flexible cable by injecting a calibration signal at one end of the test signal cable and measuring a reflection signal generated by a mismatch at the other end of the cable. A short-circuited directional coupler is used to provide the mismatch while also providing a proper match for the moving antenna connected at the other end of the cable. Scattering parameters, the input reflection coefficient, and other parameters are measured by a network analyzer to determine the phase change. The phase error determined in this manner is then removed from the antenna measurement data associated with the AUT by post processing of the measurement data.
Both the Newell and Tuovinen measurement schemes rely upon the measurement of a reflection signal to determine the phase errors for a cable in question. However, it is well known that signal reflections other than the desired reflection signal are generated by other discontinuities in a cable. Consequently, the measurement of the reflection signal also includes the measurement of spurious return signals that corrupt the measurement of the desired reflection signal. Furthermore, the desired reflection signal, which is produced by reflecting the calibration signal from a mismatch located at the far end of the cable, is greatly attenuated for a long cable length because the signal traverses the cable twice (down to the mismatch and back) prior to measurement, effectively providing a two-way signal path. For the Tuovinen measurement technique, the presence of a leakage signal, introduced by the directional coupler as a result of the insufficiently high directivity of the directional coupler, further complicates an accurate measurement of the desired reflection because the leakage signal may have a larger amplitude than the attenuated reflection signal for a long cable length. In addition, the Newell and Tuovinen measurement techniques are limited to the measurement of phase errors, and not amplitude errors, induced by environmental influences upon an electrical cable.
To accurately compensate antenna measurement data acquired via the test signal cable, the Newell and Tuovinen techniques require measurements to determine the phase error induced by environmental influences upon the test signal cable. However, the rate of cable variation may exceed the rate of calibration data acquisition associated with the measurements of the phase error by the use of conventional instrumentation. Accordingly, it would be beneficial to automate the measurements of the phase error by the use of a controllable measurement instrument and to increase the speed of calibration data acquisition.
New measurement receivers having multiple measurement channels are now available that perform automatic measurements and computations at a significantly higher data acquisition and processing rate than available from other currently available conventional systems. In particular, the Model 1795 microwave receiver marketed by Scientific Atlanta, Inc., assignee of the present invention, provides the automatic measurement of both phase and amplitude data from the test signal cable to increase the measurement acquisition speed for the calibration measurements. Such an automated microwave receiver leads to the requirement for a method to determine the errors induced by cable variation in a manner that takes complete advantage of the automated measurement and processing capabilities of the Scientific Atlanta 1795 receiver.
Accordingly, there is a need for measurement methods that permit the errors induced by environmental influences upon an electrical device to be measured in a one-way transmission path without introducing the spurious reflection signal or leakage signal difficulties resulting from use of a two-way signal path. In addition, there is a need for a method of determining such errors to permit the systematic calibration of measurements acquired from the electrical device. There is also a need for a system for automatically determining errors induced by environmental influences upon an electrical device, such as an electrical cable, and for providing a correction factor to enable the correction of measurement data acquired from the electrical device. In particular, there is a need for a system for automatically determining errors induced by cable flexure, changes of ambient temperature, and other environmental influences upon an electrical cable.
Furthermore, there is a need for a system that provides a correction factor that can be automatically applied to measurement data acquired from an electrical cable and thereby enable the correction of cable variations for such measurement data. There is also a need for automatically determining errors attributable to cable flexure, changes of ambient temperature, and other mechanical and/or environmental influences upon an electrical cable and, furthermore, for providing a correction factor to correct measurement data acquired by the electrical cable without a priori knowledge concerning the electrical characteristics of the electrical cable. There is also a general need for automatically determining and compensating for errors induced by deforming mechanical perturbations or influences such as bending, twisting, flexing, linear motion, or rotary motion, and by environmental perturbations or influences such as temperature, humidity, and fluid pressure variations, upon an electrical cable that connects a measurement instrument to a remotely located sensor.
Finally, there is a need for more rapidly determining errors induced by environmental influences upon an electrical cable utilized to carry a test signal between a moving antenna and measurement instrumentation located within a near-field antenna measurement range and, furthermore, for calculating a correction factor to provide for corrected measurements of an AUT.